object-file: drop OBJECT_INFO_ALLOW_UNKNOWN_TYPE flag

object-file: drop OBJECT_INFO_ALLOW_UNKNOWN_TYPE flag

Since cat-file dropped its "--allow-unknown-type" option in the previous
commit, there are no more uses of the internal flag that implemented it.
Let's drop it.

That in turn lets us drop the strbuf parameter of unpack_loose_header(),
which now is always NULL. And without that, we can drop all of the
additional code to inflate larger headers into the strbuf.

Arguably we could drop ULHR_TOO_LONG, as no callers really care about
the distinction from ULHR_BAD. But it's easy enough to retain, and it
does let us produce a slightly more specific message in one instance.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>